The number of Sertoli cells (SCs) ultimately determines the upper limit of sperm production in the testis. Previous studies have shown that thyroid hormones (TH) receptors are abundantly expressed in developing SCs; therefore, it was highly significant to discover that transient neonatal hypothyroidism induced by the goitrogen 6-n-propyl-2-thiouracil (PTU) can extend SCs proliferation beyond the first 2 weeks postnatal and increase testis weight and sperm production. Further studies concluded that treatment must begin before day 8 post birth in rats. Recent studies, however, showed that SCs present in the transition region at the rete testis exhibit a more immature phenotype and have prolonged mitotic activity, which led to the hypothesis that SCs in this region will retain the capacity to respond to PTU treatment over a longer period of time. In the present study, male Wistar rats were treated with PTU from days 21 to 40 and were evaluated at 40 and 160 days of age. Similar to neonatal rat SCs, it was demonstrated that prepubertal SCs in the transition region have a high mitotic activity and are highly sensitive to TH levels. This delayed, transient hypothyroidism resulted in significantly increased testis weight, SCs number and daily sperm production. The results demonstrate for the first time that Sertoli cells showing plasticity in the transition region can be stimulated to increase proliferation and contribute to a late stage surge in testis weight and sperm output.